1. Field of the Invention
The present invention relates to a plasma Generating apparatus and a surface processing apparatus, and more particularly to a surface processing apparatus for forming a thin film by using a vapor phase growth method and for etching a thin film.
2. Description of the Related Art
Conventionally, there has been the reactive ion etching (RIE) method as one of dry etching methods widely used for fine working in semiconductor device manufacturing precesses. Further, there has been the magnetron RIE method in which a magnetic field is applied to plasma generated in the RIE process so as to increase the density of the plasma, thereby to increase the etching rate and the accuracy in the fine working for semiconductor devices.
FIG. 57 illustrates an example of a conventional apparatus used for carrying out the magnetron RIE. The apparatus is provided with an anode 7 attached to an upper inner wall of a vacuum container 1, and a cathode 2 disposed opposite to the anode 7 and also functioning as a support for a substrate (wafer) 3 to be precessed. Electric power generated at a high frequency power source 5 is applied across the anode 7 and the cathode 2 through a matching circuit 14. The resulting electric field generates a plasma in a region between these electrodes. By a self-biasing electric field induced on the surface of the cathode 2, reactive ions in the plasma are accelerated to cause the ions to impinge on the substrate to thereby progress an etching reaction. In the magnetron RIE, a magnetic field generated by a magnet 10 is applied to the self-biasing electric field in a direction perpendicular to the direction of the electric field. In FIG. 57, magnetic force lines 11 are schematically shown. By intersecting the electric field E with the magnetic field B at the right angle, the electrons in the plasma are drifted by Lorentz force in the direction of E.times.B, which causes the electrons to run in the plasma over a long distance. As a result, the frequency of the electrons to impinge on neutral molecules and atoms increases so that the plasma density increases. Further, by applying the magnetic field to the plasma, the electrons are confined to within the plasma, which prolongs the lifetime of the electrons (the period of time until the electrons finally impinge on the chamber wall, electrode or substrate) and hence increases the plasma density.
When the plasma density is increased, not only the etching rate is increased, but also the directivity of the ions is increased. In addition, an ion energy which would increase damage and decrease the etch selectivity can be maintained sufficiently low even when the gas pressure is reduced to suppress the reaction of neutral species and a film to be etched (isotropic reaction).
As described above, the magnetron RIE exhibits excellent performances. Therefore, it has been used in the processing of various thin films. However, with the magnet used in the conventional manner, the uniformity of the etching rate is degraded because of unevenness of the strength and direction of the magnetic field generated by the magnet. Further, ions are disturbed in directivity and impinge on the wafer in various directions so that they enter the wafer surface at oblique angles at least some portions of the wafer. As a result, a desirable anisotropic etching cannot be performed and the rate of etching to a pattern having a narrow width or a high aspect ratio is decreased.
For example, as shown in FIG. 58, by the etching apparatus of FIG. 57, the wafer 3 has a satisfactory etched shape at the central portion B while the peripheral portions A and C of the wafer are obliquely etched due to oblique incident direction of the ions to thereby have undesirable anisotropic oblique configuration.
Although the mechanisms causing such undesirable etched configuration have not been known in a strict sense, they can be explained as follows.
Referring to FIG. 59, the magnetic force lines in the peripheral portion of the wafer 3 are not parallel to the surface of the wafer, but are curved and formed obliquely relative to the wafer. Since the electric field is relatively weak in the plasma compared with the magnetic field, the electrons are influenced solely by the magnetic field and move in a spiral motion having a radius of about 1 mm while surrounding the magnetic force lines. Therefore, at a place where the magnetic force lines intersect with the wafer 3, electrons enter obliquely into the wafer along the magnetic force lines. Since the mass of ions directly involving the etching reaction is large, the moving direction of the ions is not greatly bent by the magnetic field. However, when electrons enter obliquely into grooves in the wafer under etching, they impinge on only one side of the groove walls. As a result, the quantities of the electric charges on both sides of the groove walls are not equal. Thus, another electric field occurs across both sides of the groove walls, which acts on the ions to bend its moving direction so that undesirable etched shape is produced.
It is known that when a substrate on which devices having a MOS structure are formed is processed, such uneven distribution of electric charges on the wafer surface would cause insulation break down through a thin insulating film such as a gate oxide film or increase in the leakage current.
Since the etching apparatus of FIG. 57 uses a leakage flux of the magnetic field, a magnet producing a very large magnetic field must be used to obtain a required magnetic field on a substrate to be processed. However, a magnet producing a large magnetic field has a very large weight so that it is very difficult to assemble the etching apparatus. Each time the material of a substrate to be processed is changed, the magnet must be changed so as to change the distribution and strength of the magnetic field on the substrate. A large magnetic field produced by the magnet influences on broad regions surrounding the etching apparatus. Therefore, electronic devices sensitive to magnetism cannot be used in the surrounding broad regions. There is a multi-chamber system in which an apparatus has a plurality of reactive chambers each having a magnet. In the multi-chamber system, leakage flux from the plurality of reactive chambers interfere with each other to distort each magnetic field in the reactive chamber, which influence the process greatly. Therefore, the conventional apparatus utilizing the leakage flux of the magnet cannot be used in the multi-chamber system.
There has been proposed an apparatus which is provided with two coils of electromagnets at the opposing sides of a vacuum processing chamber. These two coils are disposed perpendicular to each other and supplied with alternating currents whose phases are shifted by 90 degrees so that the magnetic field produced by the electromagnets rotates. As the magnetic field rotates, however, the strength and distribution of the magnetic flux are changed so that the magnetic field is distorted. Further, as the frequency of the alternating current applied to the coils is increased to increase the rotation speed of the magnetic field, the impedance of the coils increases. Thus, there is a limit to the rotational speed. In order to supply a uniform magnetic field to the overall surface of the wafer, the coil diameter is required to be sufficiently large compared to the distance between the coils or the size of the container of the apparatus. As the diameter of the wafer increases, the size of the coil must be increased so that the current flowing through the coils and the size of a power source therefor increase.
Furthermore, since the strong magnetic field acts on regions in which no magnetic field is required inside and outside of the container, electro-mechanical parts sensitive to magnetism cannot be disposed in those regions. Further, magnetic shielding for protecting outside apparatus is required.
The above problems occur not only in the etching process, but also in all surface processing which use plasma such as depositing processes including sputtering and CVD, impurity implantation processes and surface modifying precesses, in terms of uniformity, accuracy, and damage.
As described above, since the strength and direction of the magnetic field are uneven in the conventional magnetron RIE apparatus, uniform etching is not maintained, and the directivity of ions is disturbed, so that ions enter obliquely into a substrate surface to be processed to thereby render high anisotropic etching difficult. When the magnetic field strength is tried to be increased, the magnet weight is larger and the apparatus becomes difficult to compose. Each time the distribution and strength of the magnetic field is changed, the magnet used must be changed. In addition, leakage magnetic field is large and a plurality of reactive chambers cannot be provided in close relationship.